More Windmill Tilting

 Here, I intend to discuss how badly we have screwed up by not using heat pumps when installing central air conditioning units.

Since about the 1970s, I think when heat pumps became available.  If "heat pump" does not mean much to you, think "reversible air conditioners".

Early "reversible air conditioners" did a relatively poor job of heating.  That, because the "heated space temperature" and outside air temperature difference can be only about 30-40 deg F.  With outside air temperatures around 40 deg, heated air can be no higher than about 70-80 deg.  More modern heat pumps, since about 2010-2015, can produce sufficient heat with outside air temperatures as low, or lower than, about 20 deg.  "Northern" heat pumps can go as low as about -20 deg but have higher costs than our modern "Southern" heat pumps that go down to 0-20 deg.

Heat pumps can be used instead of air conditioners (only) for an additional cost of $500-$1000.  Most of the heating hardware is present on "only cooling heat pumps".    Even old style heat pumps can produce the needed heat for outside air temperatures as low as about 40 deg.  And produce that heat FAR cheaper than the "normal" heating home heating system.

Why are modern heat pumps so desirable now for both heating and cooling?  The smaller the temperature increase desired, the less power/energy a heat pump needs to reach that temperature.  Heat pumps are most efficient when the differential is ~30 deg (+/-).   A heat pump CAN be the sole source of heat even at much greater differentials.  Overall, heat pumps are almost certain to be more  cost efficient than resistive or combustion heating.  That is even when the heat pump is operating at large differentials because those periods are short compared to the periods when heat pumps are operating more cost effectively.  A ground source heat pump can produce needed heat cost effectively in all climates and temperatures.

I've been speaking of "air source heat pumps" rather than "ground source heat pumps".  Traditionally, ground source has been considered prohibitively expensive in The South because of the expense of reaching the ~70 deg temperatures that are below ~100 feet in our area.

If The Boring Company can tunnel much more cheaply than other digging options, they could create under ground plenum chambers that could supply whole neighborhoods with ground temperature air for direct heating and cooling or as a "ground source" for heat pumps.  In this (central Texas) area, the ground temperature 100-200' deep is about 70 deg year round.  Additionally, here around Dale, the digging is easy; rocks are fairly rare.   Someone might estimate how large a cavity is needed to supply a house with ground temperature air.

For decades I've contemplated how ground water (from a well) might be used for a ground source heat pump.  I'm sure it can be done much more cheaply than boring holes dedicated to the function.  BUT, there are leakage and corrosion problems that need to be anticipated.  The problem is that apparently no one has applied themselves to the problem; all the ground source installers want to sell the expensive excavations.  In The South, I don't believe such expense can be justified.  OTOH, I've estimated that well water costs about $.10/1000gal to produce.  That is a result of my time irrigating many acres from wells.  That cost included power, water well installation costs, etc in the 1970s-1980s.  We are in a aquifer recharge zone; that means that any water put on the ground eventually finds it's way down to the aquifer rather than running off to the Gulf of Mexico.  That is, no second well to put water back into the ground is needed. 

I'm not finished with this.